Rename mesh-control -> mesh-controller, substrate -> foundation

One name per thing, per the HQ glossary: the module/container/image/binary/repo
becomes mesh-controller, the seat the-controller, and the store+broker pair the
foundation (embedded base bundles, default template and example lock renamed with
their go:embed directives). No behaviour change — a pure vocabulary rename.

Claude-Session: https://claude.ai/code/session_01D6qtiYU3P9jk3pnAXyAFyx

internal/image/image.go

@@ -58,7 +58,7 @@ var saved []byte
 var ErrEmpty = errors.New(
 	"this mesh-bootstrap carries no builder image, so it cannot raise a mesh. A release build " +
 		"embeds one: `make bootstrap IMAGE=<image>` in the mesh-host repository, where <image> " +
-		"is a mesh-builder image already built from the mesh-control source")
+		"is a mesh-builder image already built from the mesh-controller source")
 
 // IsEmpty reports whether anything was built in.
 //